1. Field of the Invention
The present invention relates to a method for patterning organic thin films that are difficult to pattern using conventional means such as lithography and etching.
2. Background of the Invention
Organic thin films such as pentacene, hexathiophene and polythiophene have been used in various applications, and continued research on such material is presently ongoing. Organic thin films are typically used in electronic devices, such as organic thin film transistors (OTFTs) and organic light emitting diodes (OLEDs). Unfortunately, many organic materials which have useful electronic properties are intolerant of solvent exposure after deposition, and therefore cannot be patterned using ordinary lithographic processes.
Researchers in this field thus normally either use unpatterned films or use a shadow mask during deposition to pattern the organic material. Recently, M. Roussy, et al. have developed an isolation technique which employs topographic discontinuities to isolate the devices (see, M. Roussy, et al. xe2x80x9cForming a Passive matrix Addressed OLED with a Lift-Off Photoresistxe2x80x9d, Abstracts of the Materials Research Society, 1998 Fall Meeting, p. 374). In this technique, the photoresist is laid down, exposed and caused to swell; the swelling creates gaps in the surface topography. A material is then vapor deposited at an appropriate angle to prevent bridging of the gaps thus formed thereby effectively isolating the devices. This technique developed by M. Roussy, et al. requires that the semiconductor material and the photoresist remain in inactive areas. This restriction can be a severe limitation for many applications. A subtractive technique for isolating devices has not been found in the prior art.
There are several disadvantages to using unpatterned films and topographic techniques to isolate the active areas in OFETs and OLEDs. The disadvantages include:
(i) In OFET and OLED applications, it is often desirable that inactive areas are optically clear; this can only be achieved by removing the material from the inactive areas.
(ii) In OFET and OLED applications, deposition of additional active layers (such as additional material to vary OLED emission color, or OLEDs onto an OFET controlled device) may be desired. This will require removal of inactive material and a flat topography.
(iii) Topography isolation techniques may only be used when the organic thin film material is deposited by a strongly directional process, such as vacuum deposition.
Sputtered or spun-on organic thin films will not be patterned.
(iv) Using unpatterned organic material in OFET applications permits fringe current flow through an ungated area, which leads to a decrease in the on/off ratio, a significant figure of merit, which in unpatterned devices adversely affects the contrast ratio in analog applications and the noise margin in digital applications.
The use of shadow masks, on the other hand, provides patterning of the material without some of these disadvantages. There are however several limitations associated with using shadow masks. These limitations include:
(i) The minimum feature size definable by a shadow mask is much larger than that which can be lithographically processed.
(ii) Shadow masks may only be used in vapor deposited processes, such as molecular beam deposition; spun-on material cannot be patterned in this way.
(iii) Use of a shadow mask requires contact with the substrate; this can introduce scratches and other undesirable defects to the electronic device.
(iv) The edge of devices created using shadow masks are susceptible to adhesion to the mask; creating poorly defined edges.
(v) Thin and fragile masks must be used due to the small dimensions involved; such masks often develop defects and break quickly.
(vi) Shadow masks must be cleaned; this is time consuming and often damages the mask.
(vii) Because the mask is so thin, large areas cannot be covered with a shadow mask.
It is also possible in the prior art to pattern organic thin films using a photoresist. In such techniques, the photoresist is applied directly to the surface of an organic thin film and then patterned using lithography and etching. A major drawback of this technique is that the photoresist and the solvent it contains as well as the etchants used come into direct contact with the organic thin film. Such direct contact can cause damage or alter the structure and properties of some organic films.
In view of the numerous drawbacks with the above mentioned prior art processes, there remains a need for developing a method in which organic thin films can be patterned without damaging the surface of the organic thin film.
One object of the present invention is to provide a subtractive etch patterning method for patterning organic thin films which are difficult to etch using conventional lithography and etching processes and do not tolerate contact with solvents and other chemicals without resulting in damage to the organic thin film or alteration of its structure and properties.
Another object of the present invention is to provide a method for isolating active areas of organic thin films not tolerant of solvent or other chemical exposure.
A yet other object of the present invention is to provide a method which uses substantially subtractive means to perform the isolation, leaving other areas free of material.
Other objects of the present invention include:
To provide an isolation method which operates independently of the method of deposition of the organic thin film.
To provide an isolation method which operates independently of the chemical resistance of the organic thin film material and the composition of the photoresist used.
To provide an isolation method which requires no contact with the substrate or the use of a fragile component.
These and other objects and advantages can be obtained in the present invention by utilizing a method wherein a protective material layer is employed and is used to protect an underlying chemically sensitive organic thin film. It is noted that the term xe2x80x9cthinxe2x80x9d, when used in conjunction with the chemically sensitive organic thin film, denotes a thickness about 10,000 xc3x85 or less. The term xe2x80x9cchemically sensitivexe2x80x9d denotes a material that is highly responsive, i.e., susceptible, to solvent, etchant or other chemical exposure such that upon contact with the chemical it may be damaged, while the term xe2x80x9cchemically resistantxe2x80x9d denotes a material that is not susceptible to damage or alteration of properties and structure by exposure to most chemicals and especially the ones used in the patterning process. However, it should be etchable with reactive-ion etching (RIE) or any other dry etching processes.
Specifically, the method of the present invention comprises the steps of:
(a) forming a protective material layer on the surface of a chemically sensitive organic thin film, said protective material layer being chemically resistant;
(b) forming a photoresist on an exposed surface of said protective material layer;
(c) patterning the photoresist; and
(d) transferring the pattern to the protective material layer and the chemically sensitive organic thin film by dry etching.
The method of the present invention can be used in conjunction with typical processing steps that are used in fabricating OTFTs and OLEDs.